Archaeologists searching Portugal's coast have found a 400-year-old shipwreck believed to have sunk near Lisbon after returning from India laden with spices.

Project director Jorge Freire said from a heritage perspective the vessel was the "discovery of the decade".

"In Portugal, this is the most important find of all time," he said.

In and around the shipwreck, 12 metres below the surface, divers found spices, nine bronze cannons engraved with the Portuguese coat of arms, Chinese ceramics and cowry shells, a type of currency used to trade slaves during the colonial era.

Found on September 3 off the coast of Cascais, a resort town on the outskirts of Lisbon, the shipwreck and its objects were "very well preserved", Mr Freire said.

Mr Freire and his team believe the ship was wrecked between 1575 and 1625, when Portugal's spice trade with India was at its peak.

In 1994, Portuguese ship Our Lady of the Martyrs was discovered near Fort of Sao Juliao da Barra, a military defence complex near Cascais.

"For a long time, specialists have considered the mouth of the Tagus river a hotspot for shipwrecks," Minister of Culture Luis Mendes said.

"This discovery came to prove it."

The wreck was found as part of a 10-year-old archaeological project backed by the municipal council of Cascais, the Navy, the Portuguese Government and Nova University of Lisbon.